The fuel efficiency of automobiles has been conventionally improved by reducing the rolling resistance of tires (in other words, by improving the performance in terms of rolling resistance). As automobiles have been required to have still more improved fuel economy recently, rubber compositions for tire components have been required to be excellent in low heat build-up. Therefore, rubber compositions with a low tan δ and low heat build-up have begun to be used in, for example, the tread portion or the sidewall portion.
Meanwhile, in the inner liner portion inside a tire, butyl rubbers with low air permeability are generally used in order to retain air. These butyl rubbers, however, have higher hysteresis losses than those of diene rubbers used in the tread portion and the sidewall portion. Thus, it is difficult to readily prepare an inner liner rubber composition with a low tan δ.
The tan δ of an inner liner rubber composition can be lowered by reducing the amount of carbon black; however, the viscosity of the unvulcanized rubber composition is then reduced correspondingly, leading to difficulty in controlling the rubber thickness in the molding process, which means deteriorated sheet processability. In this context, Patent Literature 1 discloses that the balance between the reduction in tan δ and the retention of sheet processability can be improved by partially replacing carbon black with finely ground bituminous coal. Meanwhile, Patent Literature 2 discloses that the moisture barrier properties of tires are improved by using in an insulation a rubber composition that includes a rubber component containing a large amount of natural rubber in combination with finely ground bituminous coal. However, Patent Literatures 1 and 2 still leave room for improvement in improving handling stability, fuel economy, elongation at break, sheet processability, air barrier properties, and compounding cost in a balanced manner.